The invention relates to apparatus for recording and/or reading information on an optically readable disc which is rotatable about an axis of rotation, which apparatus comprises: a frame; a slide which is movable over the frame along a radial path relative to the axis of rotation; electro-dynamic slide drive means for driving the slide along the radial path; an objective which is arranged on the slide and which is movable relative to the slide along a focussing axis parallel to the axis of rotation; electro-dynamic focusing drive means for driving the objective along the focusing axis; and an objective bearing arrangement comprising bearing means connected to the slide and bearing means connected to the objective.
Apparatus of the type described above is known in various versions, for example as optical video-disc players, optical audio-disc players or peripheral computer equipment for the storage and reproduction of the information on optical storage discs. In all these devices the objective is movable along the focusing axis in order to enable movements of the information surface, which inevitably occur during rotation of the disc, to be followed accurately with the laser beam which is formed to a read spot. This is necessary because the depth of focus of the objectives used is of the order of magnitude of a few microns only. Therefore, the slide must be provided with a bearing arrangement which allows the focusing movements of the objective and, depending on the construction of the apparatus, radial movements for following the track and tangential movement for correcting time errors. The objective bearing arrangement inhibits undesired movements of the objective. The radial travel of the slide on which the objective is arranged is such that the read spot can reach every track on the optically readable disc. For the rapid location of a track--and as the case may be rapidly finding a blank part of the optically readable disc in order to record information in this part--it is important that the slide can be moved rapidly along its radial path. Since the objective has a certain mass radial forces are exerted on the objective during the fast radial movements of the slide, which forces may be of such an amplitude that problems may arise when the objective is moved along the focusing axis simultaneously. Depending on the type of objective bearing arrangement used these problems may be friction problems and bandwidth problems. This will now be explained in more detail with reference to some known devices of the type mentioned in the opening paragraph.
The Applicants previous Patent Application No. 8204981, which has not been published in due time, describes a device in which the objective is arranged on the slide so as to be movable by means of a plurality of plate springs which extend in the same direction as the path of the slide. The bandwidth for the radial slide drive is limited because at high frequencies the plate springs are no longer sufficiently rigid in the radial direction. The deformation of the plate springs then gives rise to radial movements of the objective relative to the slide.
It is also known, for example from U.S. Pat. No. 4,021,101, to use a sleeve-bearing arrangement for the objective. The objective is movable in a bearing sleeve, which inevitably gives rise to some friction. In the case of slow or uniform radial movements of the slide this friction is very low because no forces act on the objective in the radial direction and, consequently, in a direction normal to the friction surfaces of the objective bearing arrangement. This is different if the slide must be moved with high accelerations and decelerations. The frictional forces then occurring depend directly on the accelerations and decelerations of the slide movement. Both the movements of the objective along the focusing axis and the movements of the slide along the radial path are controlled by electronic servocircuits. The variable friction in a focussing bearing arrangement gives rise to cross-talk between the two servo-circuits. Another problem is caused by the radial clearance between the objective and the sleeve. Tracking becomes more difficult when radial forces are exerted on the objective because then the objective does not occupy a well-defined position in the sleeve but is urged either against one wall or against the other wall, depending on the direction of movement of the slide. This may be precluded by providing a prestress in the objective bearing arrangement. However, this leads to an increase of the friction in the objective bearing arrangement, which is undesirable for several reasons, while the modulation of the frictional force as a result of the accelerations and decelerations of the slide persists.